A motor can be regarded as a multi-functional component which includes a rotor, a shaft, a bearing, a stator, and the like obtained by highly precisely processing various materials such as steel, non-ferrous metal, and polymer and which converts electric energy into mechanic energy by the combination thereof. In recent years, a so-called permanent magnet type motor which uses a magnet capable of attracting or repelling other magnetic materials and of permanently generating a static magnetic field without an external energy is widely used. From the viewpoint of physics, the magnet is different from other magnetic materials in that an effective magnetization remains even after removing an external magnetic field. When a heat or a comparatively large inverse magnetic field is applied, a magnetization inversion (demagnetization) eventually occurs and then the magnetization reduces with the demagnetization. An important characteristic value of the magnet is an energy density (BH) max. The energy density (BH) max shows potential energy of the magnet by the unit of volume.
Incidentally, the performance of the strong attracting or repelling capability of the magnet does not contribute to improve the high performance of every type of the motor. However, in Non-patent Document 1, on the basis of a relationship between a residual magnetic flux density Br corresponding to one of basic characteristics of the magnet and a motor constant KJ (KJ is a ratio between an output torque KT and a square root √R of a resistance loss) corresponding to an index of a motor performance, it is described that an increase in energy density (BH) max of the magnet induces a higher torque density in the radial-direction gap type magnet motor, which is a target of the invention, in the state where a motor diameter, a rotor diameter, a gap, a soft magnetic material, a magnet dimension, and the like are fixed.
However, the increase in energy density (BH) max of the magnet induces the higher torque density in the radial-direction gap type magnet motor, which is a target of the invention, but since a stator iron core of the motor is provided with teeth forming a part of a magnetic circuit and a slot accommodating wound wires, the permeance changes with the rotation. For this reason, the increase in energy density (BH) max of the magnet increases a torque pulsation, that is, a cogging torque. The increase in cogging torque causes bad influences such as the disturbance of a smooth rotation of the motor, the increase in vibration or noise of the motor, and the deterioration in rotation control performance.
In order to avoid such bad influences, many studies on the cogging torque reduction of the radial-direction gap type magnet motor have been carried out in the past.
First, regarding the magnetic pole having an even thickness in a magnetization direction, making the thickness of the magnet uneven is considered. For example, in Non-patent Document 2, it is described that the cogging torque becomes minimum when the radial-direction gap type magnet motor shown in FIG. 9A including magnetic pole 1 of an uneven thickness, stator iron core 2, stator iron core slot 3, and stator iron core teeth 4 is allowed to have the 12-pole-18-slot uneven-thickness magnetic pole with a residual magnetization Br of 1.2 T, a maximum thickness at the magnetic pole center of 3 mm, and a minimum thickness at both magnetic pole ends of 1.5 mm. Additionally, in this case, the thickness is unevened from the outer diameter side of the magnetic pole. However, it is known that the cogging torque can be reduced even in the magnetic pole whose thickness is unevened from the inner diameter side of the magnetic pole.
In addition, in Non-patent Document 2, in order to minimize the cogging torque by making the thickness of the magnetic pole uneven as shown in FIG. 9A, the minimum thickness of both magnetic pole ends is required to be unevened so that the thickness is about a half of the maximum thickness of the magnetic pole center. Accordingly, if the thickness of the magnetic pole, that is, the magnetization direction (thickness) becomes thinner, a sufficient advantage cannot be obtained even when the thickness of the magnetic pole is unevened so as to minimize the cogging torque. In addition, the magnetic pole is mechanically weak in general, and thus the processing thereof becomes difficult.
Meanwhile, regarding the magnetic pole whose thickness is thin in the magnetization direction, there is known a method of skewing the magnetic pole as disclosed in FIG. 9B in Non-patent Document 3 or a method of continuously removing a magnetic pole area between magnetic poles as disclosed in FIG. 9C in Non-patent Document 4.
According to the summary of the known techniques described above, the magnetic pole end of the thick magnetic pole is thinned by about a half so as to broaden a gap between itself and the stator iron core or the area between the magnetic poles of the thin magnetic pole is reduced. Accordingly, the amount of the static magnetic field Ms, generated from the magnetic pole and flowing through the stator iron core in the form of the magnetic flux Φ, is suppressed. As a result, in theses methods, the torque density decreases by 10 to 15% in general due to the reduction in cogging torque. Accordingly, the cogging torque reduction methods using the known techniques shown in FIGS. 9A, 9B, and 9C are contrary to the technique in which the increase in energy density (BH) max of the magnet induces the increase in torque density in the radial-direction gap type magnet motor.
Meanwhile, as in Non-patent Document 5, a cogging torque reduction method of the radial-direction gap type magnet motor is reported which adopts an Nd2Fe14B-based rare-earth sintered magnet whose thickness in the magnetization direction is as thin as 1.2 mm and the residual magnetization Mr has a high energy density of 1 T. In this cogging torque reduction method, as shown in FIGS. 9A, 9B, and 9C, the thickness in the magnetization direction or the magnetic pole area is not decreased. That is, as shown in FIGS. 10A to 10D, a so-called Halbach Cylinder is shown each magnetic pole of which is formed of two to five divided sections, and the magnetization direction (anisotropic direction) for each divided section is adjusted stepwisely. Here, in the drawing, the suffixes (2) to (5) indicate the number of two to five divided sections of one magnetic pole. In addition, the direction indicated by the arrow of each divided section indicates a direction of the magnetization vector M along the oriented magnetization easy axis (C-axis), that is, the anisotropic direction.
When the 12-pole-18-slot radial-direction gap type magnet motor is manufactured by adopting the magnetic pole having the above-described configuration, the plotted cogging torque with respect to the number of divided magnetic pole sections of the magnetic pole is shown in FIG. 11. That is, the number N of the divided magnetic pole sections of the magnetic pole and the cogging torque Tcog satisfy the exponential approximation as Tcog=61.753 exp (−0.1451 N). In addition, FIG. 11 shows that the state where Mθ/φp minutely and continuously changes in a specific direction particularly between different poles is ideal when Mθ denotes an angle formed between the magnetization vector M at an arbitrary mechanic angle φ. However, in the Nd2Fe14B-based rare-earth sintered magnet whose thickness is 1.2 mm and whose residual magnetization Mr has the high energy density of 1 T, it is difficult to prepare plural magnetic pole sections having different anisotropic directions, to minutely and regularly arrange the magnetic pole divided sections, and to form the magnetic pole with high dimensional precision. For this reason, it is very difficult to manufacture a multi-pole rotor having an integer number of the magnetic poles and a radial-direction gap type magnet motor adopting the multi-pole rotor. In addition, it is easily supposed that manufacturing such multi-pole rotor or a radial-direction gap type magnet motor is not compatible with economical efficiency.
An object of the invention is to provide a new cogging torque reduction method of a radial-direction gap type magnet motor which does not reduce the volume or area of the magnetic pole of, for example, a magnetic anisotropic magnetic pole whose thickness is as thin as 1.5 mm, and is difficult to be unevened, and whose energy density is high.
More specifically, the present invention took note of the disclosure in Non-patent Document 5 that the number N of divided sections of the magnetic pole and the cogging torque Tcog satisfy the exponential approximation as Tcog=61.753 exp (−0.1451 N). Particularly, when φp denotes the mechanic angle of the magnetic pole and Mθ denotes an angle formed between the circumferential tangential line of the magnetic pole and the magnetization vector M in the region of φp×0.1° at a position between different poles in which N and S poles are switched, it is an object of the invention to simultaneously obtain the contrary advantages, that is, the suppression of the cogging torque and the increase in torque density using the magnetic anisotropic magnetic pole having the high energy density by specifying the continuous change Mθ/φp of the magnetization vector angle Mθ with respect to the mechanic angle φp of the magnetic pole. Accordingly, it is possible to provide the radial-direction gap type magnet motor which adopts the magnetic anisotropic magnetic pole having the energy density (BH) max≧150 kJ/m3 and which simultaneously realizes the contrary advantages, that is, the suppression of the cogging torque and the increase in energy density.
A radial-direction gap type magnet motor according to the invention includes a magnetic anisotropic magnetic pole in which assuming that φt denotes a mechanic angle of a stator iron core teeth of the radial-direction gap type magnet, φp denotes a mechanical angle of a magnetic pole, and Mθ denotes an angle of a magnetization vector with respect to a circumferential tangential line of the magnetic pole, Mθ of a mechanical angle region φp=φt of a magnetic pole center region facing φt is set to be 75 to 90° and more desirably 90° so as to have an average error within 5°, and Mθ/φp≦7 is satisfied in the region of φp×0.1° at a position of a circumferential magnetic pole end, that is, a position between different poles. The Mθ and φp have precision at which the linear approximation having the correlation coefficient of 0.99 or more is satisfied.
In the magnetic anisotropic magnetic pole, when Hθ denotes an angle formed between the direction of the homogeneous external magnetic field Hex and the tangential line of the mechanic angle φp at the inner and outer surfaces of the magnetic pole, first, a deformed magnetic pole having the inner and outer circumferential sections causing the angle change Hθ/φp is prepared by the application of the orientated magnetic field using the homogeneous external magnetic field Hex. Next, the deformed magnetic pole is subjected to a heat and an external force to thereby obtain a predetermined circular arc magnetic pole. Then, the obtained circular arc magnetic pole is applied with the homogeneous external magnetic field Hex again in the same direction as the oriented direction so as to be magnetized. At the time when the homogeneous external magnetic field Hex is applied to the circular arc magnetic pole, the respective parts of the magnetic pole are magnetized in the direction of the magnetization easy axis (C-axis). Accordingly, the magnetization vector angle Mθ with respect to the circumferential tangential line of the circular arc magnetic pole is equal to Hθ with a certain degree of error.
In order to minimize the error between the Mθ and Hθ, the shape of the deformed magnetic pole is desirably obtained in such a manner that a rigid body forming the deformed magnetic pole and having the arbitrary Hθ/φp moves in rotation, only the direction of the magnetization easy axis (C-axis) changes without breaking down the anisotropic degree, and the nonlinear structure analysis of the rigid body aggregation is carried out. In addition, the rotary movement of the rigid body aggregation having the arbitrary Hθ/φp, where only the direction of the magnetization easy axis (C-axis) changes without breaking down the anisotropic degree, is carried out by using the rheology of the molten linear polymer such as a shear flow action, an extension flow action, a viscous deformation having the shear flow action and the extension flow action caused by the heat, the external force, and the like. In addition, it is desirable that the magnetic pole has the magnetic performance such of the residual magnetization Mr≧0.95 T, the intrinsic magnetic coercive force HcJ≧0.9 MA/m, and the energy density (BH) max≧150 kJ/m3.
As a configuration of the magnetic anisotropic magnetic pole suitable for ensuring the magnetic performance of the energy density (BH) max≧150 kJ/m3 and plastic workability, for example, a macro structure is provided in which Nd2Fe14B-based rare-earth magnet particles each having an average particle diameter of 150 μm or less are isolated by a matrix (continuous phase) of a bonding agent and Sm2Fe17N3-based rare-earth magnet fine powders each having an average particle diameter of 3 to 5 μm. Desirably, the volume ratio of a magnet material having an energy density (BH) max≧270 kJ/m3 to the magnetic pole is set to be 80 vol. % or more.
A magnetic isotropic magnet can be freely magnetized in a direction of a magnetization field and in any direction in accordance with a magnetic field strength distribution. For this reason, it is possible to provide a magnetization pattern indicated by the circular arc arrow in magnetic pole 1 of FIG. 12 by optimization of the shape of a magnetization yoke and a magnetomotive force. Accordingly, it is possible to easily adjust a gap magnetic flux density distribution between a magnetic pole and a stator iron core to a sine wave shape. Thus, the cogging torque reduction in the radial-direction gap type magnet motor can be easily carried out compared with the case where a thin magnetic pole is formed of a magnetic anisotropic magnet material.
A study on the isotropic rare-earth magnet material seems to be started by R. W. Lee and others who reported that an isotropic Nd2Fe14B-based bond magnet having the energy density (BH) max of 72 kJ/m3 can be formed when a rapidly-solidified ribbon having the energy density (BH) max of 111 kJ/m3 is fixed by a resin (see “Hot-pressed Neodymium-Iron-Boron magnets” written by R. W. Lee, E. G. Brewer, and N. A. Schaffel, IEEE Trans. Magn., Vol. 21, 1958 (1985)). Since then, studies on the isotropic rare-earth magnet material mainly obtained by the rapid solidification of the rare-earth-iron-based molten alloy have been actively carried out from the late in 1980s. For example, there are many isotropic magnetic materials of different powder shapes to be industrially usable, such as Nd2Fe14B, Sm2Fe17N3, or a nanocomposite magnet material obtained by using an exchange bonding based on a microscopic structure of αFe, FeB, and Fe3B with Nd2Fe14B and Sm2Fe17N3, in addition to isotropic magnet materials obtained by the micro control of various alloy structures. For example, see Non-patent Documents 6 to 10. Particularly, in Non-patent Document 10, H. A. Davies et al. proposed a material having an isotropy and an energy density (BH) max of 220 kJ/m3.
However, the energy density (BH) max of the isotropic magnet materials which can be used in industries is 134 kJ/m3 at best. The energy density (BH) max of the isotropic Nd2Fe14B-based bond magnet which is generally used in the magnet motor represented as a small-sized radial-direction gap type magnet motor having a power of 50 W or less is approximately 80 kJ/m3 or less. That is, although more than 20 years have passed since the isotropic Nd2Fe14B-based bond magnet having the energy density (BH) max of 72 kJ/m3 was formed from the ribbon having the energy density (BH) max of 111 kJ/m3 by R. W. Lee and others in 1985, the improvement in energy density (BH) max is less than 10 kJ/m3.
Accordingly, it cannot be expected to increase the energy density and torque density of the radial-direction gap type magnet motor which is a target of the invention by depending on the slow improvement of isotropic magnetic materials.
Meanwhile, changing of an isotropic magnet to an anisotropic magnet is usually accompanied with the increase of energy density (BH) max. For this reason, in the radial-direction gap type magnet motor which is a target of the invention, a higher torque density can be obtained, but the cogging torque increases.    Non-patent Document 1: “Application of high performance magnets for small motors” written by J. Schulze, Proc. of the 18th international workshop on high performance magnets and their applications, 2004, pp. 908-915    Non-patent Document 2: “Comparison of brushless motors having halbach magnetized magnets and shaped parallel magnetized magnets” written by Y. Pang, Z. Q. Zhu, S. Ruangsinchaiwanich, and D. Howe, Proc. of the 18th international workshop on high performance magnets and their applications, 2004, pp. 400-407    Non-patent Document 3: “Properties and applications of high performance magnets” written by W. Rodewald, W. Rodewald, and M. Katter, Proc. of the 18th international workshop on high performance magnets and their applications, 2004, pp. 52-63    Non-patent Document 4: “Investigation of Increase in Performance of Blowing Brushless DC Motor” written by Atsushi Matsuoka, Togo Yamazaki, and Hitoshi Kawaguchi, Rotating Equipment Seminar of Electric Association, The Institute of Electrical Engineers of Japan (IEEJ), RM-01-161, 2001    Non-patent Document 5: “Application of halbach cylinders to electrical machine” written by D. Howe and Z. Q. Zhu, Proc. of the 17th int. workshop on rare earth magnets and their applications, 2000, pp. 903-922    Non-patent Document 6: “Development Tendency of High-performance Rare-earth Bond Magnet” written by Takahiko Iriyama, Ministry of Education, Culture, Sports, Science and Technology, Innovation Creation Project/Symposium of Efficient Usage of Rare-earth Resource and Advanced Material, 2002, pp. 19-26    Non-patent Document 7: “Recent developments in Nd—Fe—B powder” written by B. H. Rabin, and B. M. Ma, 120th Topical Symposium of the Magnetic Society of Japan, 2001, pp. 23-28    Non-patent Document 8: “Recent powder development at magnequench” written by B. M. Ma, Polymer Bonded Magnets 2002, 2002    Non-patent Document 9: “Structure and magnetic properties of Nd2Fe14B/FexB-type nanocomposite permanent magnets prepared by strip casting” written by S. Hirasawa, H. Kanekiyo, T. Miyoshi, K. Murakami, Y. Shigemoto, and T. Nishiuchi, 9th Joint MMM/INTERMAG, FG-05, 2004    Non-patent Document 10: “Nanophase Pr and Nd/Pr based rare-earth-iron-boron alloys” written by H. A. Davies, J. I. Betancourt, and C. L. Harland, Proc. of 16th Int. Workshop on Rare-Earth Magnets and Their Applications, 2000, pp. 485-495